This invention relates to altering the velocity of gaseous molecules utilizing a moving supersonic nozzle.
Many devices and techniques for manipulating neutral atoms or molecules require for their effectiveness that the translational velocity (or kinetic energy) of the particles be markedly reduced. With respect to atoms, the advent of powerful methods for cooling, trapping, and manipulating them with laser light has led to dramatic achievements, including Bose-Einstein condensation of atomic vapor, an atom laser, atom interferometry, and atom lithography. S. Chu, Rev. Mod Phys. 70, 685 (1998); C. N. Cohen-Tannoudji, Rev. Mod. Phys. 70, 707 (1998); W. D. Phillips, Rev. Mod. Phys. 70, 721 (1998). However, many such optical manipulation methods effective for atoms fail for molecules because of the complexity of the energy level structure, with its myriad vibrational and rotational components.
At ordinary temperatures, gas molecules dash about with the speed of rifle bullets. For neutral, unionized molecules, the kinetic energy arising from this thermal motion usually greatly exceeds the interaction energy of feasible external electric or magnetic fields or light fields with the molecules. Thus, while such fields can create an attractive interaction for molecules traversing some spatial region, the molecules cannot be confined there unless much of their kinetic energy is removed.
As an example, an electrostatic storage ring for dipolar molecules has been proposed. D. P. Katz, J. Chem. Phys. 107, 8491 (1997). This device, modeled on a neutron storage ring, would employ an inhomogeneous hexapolar toroidal electric field. Within the toroidal ring, molecules with suitably oriented dipoles would follow orbits determined by their rotational state and translational velocity. Design calculations limited to practical parameters indicate that storage lifetimes of the order of 103-104 seconds might be achieved. However, since the molecular trajectories must bend to stay in the ring, only molecules with low translational kinetic energy can be stored. The same constraint pertains to several other schemes for trapping or manipulating molecules. B. Friedrich and D. Herschbach, Phys. Rev. Lett. 74, 4623 (1995); H. J. Loesch, Chem. Phys. 207, 427 (1996); T. Seideman, J. Chem. Phys. 106, 2881; 107, 10420 (1997); D. R. Herschbach, in Chemical Researchxe2x80x942000 and Beyond: Challenges and Visions, P. Barkan, Ed. (Am. Chem. Soc., Washington, D.C. and Oxford Univ. Press, New York, 1998), p. 113.
At present there are only a few means by which to produce translationally cold molecules. One method involves the recombination of cold atoms into molecules using either three-body collisions or photoassociation. A. Fioretti, D. Comparat, A. Crubllier, 0. Dulieu, F. Masnon-Seeuws, and P. Pillet, Phys. Rev. Lett. 80, 4402 (1998). Although this method produces extremely slow molecules (≅300 xcexcK), the number of molecules at present is very small and the technique is limited to optically accessible, trappable atoms. A more recently demonstrated technique involves the use of time-varying electric field gradients to slow molecules via the force exerted by the transition of the molecule from high to low field. The electric fields must be switched on and off in such a way as to make sure the molecules feel only these transitions. Each such transition removes some kenetic energy from the molecules. A single such transition has been used to further cool cesium atoms liberated from an atom trap. J. Maddi, T. Dinneen, and H. Gould, Phys. Rev. A. (in press). A more striking example involved the use of 63 synchronously pulsed electric fields to slow metastable CO molecules from 225 m/s to 98 m/s. H. Bethlem, G. Berden, and G. Meijer, Phys. Rev. Lett. 83, 1558 (1999). Only molecules which are in-phase with the time-varying field are slowed (about 1% in the CO case); the rest of the molecules are virtually unaffected. For molecules which do not posses a permanently aligned dipole moment or are initially traveling faster, many more electric fields are required. At present, the most successful technique for cooling molecules is quenching their kenitic energy by collisional relaxation with a cold buffer gas. J. M. Doyle, B. Friedrich, J. Kim, and D. Patterson, Phys. Rev. A 52, R525 (1995). This technique has enabled successful trapping of the CaH molecule in a magnetic field. J. D. Weinstein, R. deCarvalho, T. Guillet, B. Friedrich, and J. Doyle, Nature 395, 148 (1998). The technique involves using the unusual isotope of helium, 3He, maintained by a dilution refrigerator at about 0.24xc2x0 K. The helium vapor density must be sufficient for collisional quenching. This technology requires that experiments be performed within a cryogenic refrigerator which is a major limitation on flexibility and scope. The cryogenic equipment, as well as provision for recycling the 3He vapor, is also quite expensive.
There is therefore a need for method and apparatus that can provide a continuous, high intensity source of molecules slowed to the range of a few meters per second (equivalent to temperatures below 1 Kelvin).
Although the disclosure to follow will be concerned with producing slow molecules, we note that the same device, rotated in the opposite direction, will accelerate the molecules. Since several other good methods are available for generating fast molecule beams, P. B. Moon, Charles T. Rettner, and J. P. Simons, Faraday Disc. 77, 630 (1977), we will not expand in detail on this mode. We note that in the accelerator mode the device might find application when it is desired to scan the molecular velocity continuously over a wide range. That mode of operation holds also for the decelerator mode even when the slowing is relatively modest. Our discussion of how to obtain maximal slowing of molecules in the decelerator mode thus implicitly includes these other less demanding operational variants.
In one aspect, the apparatus according to the invention for altering the translational velocity of molecules in a gas comprises a source of the gas and a supersonic nozzle in fluid communication with the source of gas. As used in this specification, the term xe2x80x9cmoleculexe2x80x9d is defined to include atoms and molecules. Structure is provided for moving the nozzle in a selected direction with respect to molecules emerging from the nozzle. In some embodiments, the structure moves the nozzle in a repetitive fashion, such as with a pendulum. In other embodiments, the supersonic nozzle is disposed on an arm a selected distance from an axis for rotation about the axis. The term arm is used herein to mean any structure that supports the nozzle for rotation about the axis. The nozzle has an exit portion substantially perpendicular to the arm and motive apparatus is provided for rotating the arm such that the translational velocity of the molecules exiting the nozzle is altered. It is preferred that the nozzle be oriented in a direction opposite the tangential velocity of the arm so that the translational velocity of the molecules is reduced. If desired, the nozzle can instead be oriented to increase the translational velocity of the molecules. In one embodiment, the gas flows along the axis and through a hollow arm to the nozzle. Preferably, the angular velocity of the arm is selected so that the tangential velocity of the nozzle is substantially equal to, and opposite from, the velocity of the molecules exiting the nozzle so that their resulting translational velocity (in a laboratory frame of reference) is low, in the range of a few meters per second.
In another aspect of the invention, the rotating nozzle is disposed within a vacuum chamber which may include a mass spectrometer or other means (e.g., fast ion gauge, Doppler shift of a laser induced fluorescence, a toothed wheel velocity analyzer, or other means available in the art) for recording beam molecular intensity as a function of time-of-flight of the molecules from the nozzle to the detector.
Yet another aspect of the invention is a method for altering the translational velocity of molecules in a gas including discharging the gas through a supersonic nozzle while rotating the nozzle about an axis. The rotating step may move the nozzle in a direction opposite to the direction of the gas flow so as to slow down the molecules or the rotation may move the nozzle in the same direction of the gas flow to accelerate the molecules.
The apparatus of the invention thus avoids recourse to expensive cryogenic equipment or lasers while providing a continuous, high intensity source of molecules slowed to the range of a few meters per second which is equivalent to temperatures below 1 Kelvin.